1. Technical Field
The present invention relates to a liquid droplet ejecting head and a liquid droplet ejecting apparatus, and in particular to an inkjet recording head and an inkjet recording apparatus that eject extremely small ink droplets with piezoelectric elements.
2. Related Art
In recent years, in liquid droplet ejecting apparatus represented by inkjet printers, there has been a demand to achieve a balance between high image quality recording and high speed recording. In particular, there has been a strong demand for liquid droplet ejecting apparatus that can execute recording with high image quality even with respect to plain paper in which ink bleeding and show-through easily occur.
For performing high image quality recording with respect to plain paper, using an ink whose viscosity is high (an ink whose color material concentration is high) for ensuring high recording resolution and preventing bleeding and show-through of ink is effective.
Further, in order to realize high speed recording, it is necessary to perform high resolution recording in one pass, and a so-called matrix type head has been proposed as a liquid droplet ejecting head suited for this. The ejectors of the head are arranged two dimensionally, and each ejector is connected with a common channel as shown in FIG. 7., through which ink is supplied.
However, when a high viscosity ink and a matrix type head are combined, there has been the problem that the flow path resistance in the common flow paths becomes overly large and executing stable liquid droplet ejection becomes difficult. That is, in a matrix type head, it is difficult to ensure a large cross-sectional area in the common flow paths and it is easy for the flow path resistance to become large because the common flow paths are disposed between each of the ejectors. The head size increases when the cross-sectional area of the common flow paths is increased, which becomes a problem in terms of high densification of dots and apparatus size. In addition, when a high viscosity ink is used in a common flow path whose cross-sectional area is small, the flow path resistance in the common flow path becomes extremely large; thus, sufficient ink supply can no longer be performed with respect to each of the ejectors, and it becomes difficult to execute stable liquid droplet ejection at a high frequency.
As one measure with respect to the ink supply becoming insufficient, a method of assisting the ink supply by causing the ink inside the common flow paths to circulate has also been proposed. FIG. 7 shows ejectors of a conventional matrix type head 100 and the arrangement of a common channel 114 through which ink circulation is performed. The ejectors 120 are arranged in a two dimensional matrix array. The common channels 114 are arranged between sequences of ejectors and are connected at upstream and downstream ends to common flow path mainstreams 112. Each ejector 120 is connected to the common channel 114 via a communicating path 116 (ink feed path), and ink supply to the ejectors 120 from the common channel 114 is performed. In order to aid the common channel 114 with the supply of ink to the ejectors 120, an ink circulation flow Q is created in the direction of the arrows from an upstream ejector 120A to a downstream ejector 120B.
However, in this conventional matrix type head to which ink circulation is applied, there has been the problem that variations in back pressure resulting from the flow path resistance in the common flow paths occur. As a result, realizing high image quality recording becomes impossible.
Moreover, as another problem when a high viscosity ink is used, there is the problem that ink viscosity increases in the vicinities of the nozzles. That is, in a liquid droplet ejecting head, the ejection of ink from each of the ejectors is controlled in response to a recording signal, but in a nozzle whose use frequency is low, a phenomenon arises where the ink solvent evaporates from the nozzle opening and the ink viscosity at the portion in the vicinity of the nozzle increases. When such an ink viscosity increase occurs, this causes problems such as the ejector becoming unable to perform proper ink ejection, the droplet volume and the droplet speed becoming reduced, and non-ejection occurring.
Thus, in the present invention, it is an object to provide a liquid droplet ejecting head and a liquid droplet ejecting apparatus that address the above-described problem which arises when a matrix type liquid droplet ejecting head and a high viscosity ink are combined and can achieve a balance between high image quality recording (compatible with plain paper) and high speed recording.